1. Field of the Invention
This invention relates to the use of seismic signals to delineate subsurface structures. More specifically, it relates to the use of refracted and reflected seismic signals to locate acoustic impedance boundaries such as the sides of salt domes.
2. Background of the Invention
Petroleum deposits are frequently found in traps formed around salt domes. The location of the top of a salt dome is typically mapped with seismic sources and receivers positioned at the earth's surface. See, for example, U.S. Pat. No. 4,835,745, issued May 30, 1989 to Young et al and U.S. Pat. No. 4,803,668, issued Feb. 7, 1989 to Berryhill et al.
It has been difficult, however, to map precisely the location of the interface between the sides of the salt domes and the surrounding structure. Methods known to the prior art for mapping the sides of salt domes have utilized both reflection and refraction techniques.
In the refraction method known to the prior art, normally, a seismic signal is generated at the surface over the salt dome and the resulting signal is detected in a wellbore near the side of the salt dome after the signal has travelled through a part of the salt dome. In the refraction method known to the prior art, the signal travel path was assumed to lie within a vertical plane which intersected the source and receiver locations. For each source-receiver location, the time required for the signal to travel to the receiver was measured. Previously determined information regarding the seismic velocity of the earth strata surrounding the borehole was then utilized to calculate the possible arrival paths within the assumed vertical plane of travel over a selected arc. Possible transmission paths from the source within the vertical plane were then generated, utilizing previously determined information regarding the location and shape of the top of the salt dome, the velocity profile of the strata overlying the salt dome and the velocity in the salt dome. From the previously determined velocity information, the travel time along each of the calculated paths is determinable, and locations at which a travel line from the source intersects a line of travel from the receiver, where the combined calculated travel time from the source to the intersecting location and from the receiver to the intersecting location equals the measured travel time, yields a possible travel line from source to receiver. For each source-receiver location, a plurality of intersecting points, substantially defining an aplanatic curve is determined. Each of these intersecting locations is a possible location of the salt dome boundary. In general, it was not possible to determine for a single source-receiver location which of these intersecting points represented the actual interface between the salt dome and the surrounding formation. However, by detecting the seismic signal at a large number of different depths in the wellbore, a set of aplanatic curves will be generated and an envelope of a set of such curves will described the shape of the salt face.
The reflection method of determining the interface of the sides of a salt dome is shown in U.S. Pat. No. 3,483,505, issued Dec. 9, 1969. In the reflection method, a seismic signal is initiated, either at the surface or within a wellbore adjacent the salt dome, and the signal reflected from the salt dome interface is detected by receivers in the wellbore or at the surface.
As in the refraction method known to the prior art, the travel path of the reflected signal was assumed to lie within a vertical plane intersecting the source and receiver locations. Previously determined information regarding the seismic velocity of the earth strata through which the signal travels was utilized to determine possible locations of the reflecting interface.
Because these prior art methods assumed that the entire travel paths of the seismic signal was within a vertical plane, errors were introduced when the travel path deviated from the vertical plane.
It is an object of the present invention to more accurately define the location of an acoustic impedance boundary below the earth's surface, such as the interface between a salt dome and the surrounding sediment.